1. Field of the Invention
The invention concerns a method of economically manufacturing thin refractory or ceramic flakes that can be incorporated into protective coatings for such purposes as pearlescence and resistance to abrasion and to better protect the coated substrate from corrosion.
2. Description of the Related Art
U.S. Pat. No. 3,795,524 (Sowman) discloses a method of making refractory articles (especially fibers, but also films, flakes, and microspheres) from either an aqueous solution of water-soluble aluminum and boron compounds or a colloidal dispersion of silica and water-soluble or dispersible aluminum compounds and boron compounds. Such a solution or dispersion is often called an "organosol" or a "sol-gel" or simply a "sol". To make flakes, the sol is coated onto a non-adhering smooth inert substrate and dried to form a film which may tend to crack and form flakes. After being removed from the substrate, the "film can be comminuted to form irregularly shaped particles, flakes, or aggregates, e.g., 1/16" to 1" large, 1 to 25 microns thick, or ground to powders of small size" (col. 11, ls. 39-42) that were converted to refractory flakes by being fired at temperatures up to 1000.degree. C. "In the form of particulate materials, the refractory products can be used as fillers and/or coloring agents or pigments for paints and enamels . . . ." (col. 13, ls. 8-11). They can also be used as abrasion-resistant agents for elastomeric materials (col. 13, ls. 34-40). Refractory flakes are formed in Examples 20-23 of the Sowman patent.
U.S. Pat. No. 4,349,456 (Sowman) discloses a method of making refractory microcapsules from an organosol similar to those of the earlier Sowman '524 patent. The microcapsules after being fired have diameters within the range of about 1 to 1000 .mu.m and a uniform wall thickness in the range generally of 0.1 to 100 .mu.m (col. 8, ls. 16-20). The microcapsules are crushable to flakes, as shown in FIG. 2 of Sowman '456, but we fail to find in Sowman '456 any mention of utility for these refractory flakes.
European Pat. Publ. 240952 (Saegusa), published Oct. 14, 1987, concerns a process similar to that of Sowman '524 in that a thin coating of a solution or sol of a metallic compound is applied to a smooth surface, dried, and scraped off as flakes. The flakes can either be used as such or fired at a temperature of from about 200.degree. C. to 1100.degree. C., preferably from 500.degree. C. to 900.degree. C. The resulting refractory flakes are said to be from about 0.01 to about 10 .mu.m in thickness and from about 1 to 100 .mu.m in breadth. Uses for the flakes are said to include lustrous pigments in nail enamels, enamels, leather products, exterior automotive coatings, paints, and pearl-glazed buttons. Metallic compounds used in the examples include oxides of Zr, Ti, Al, Si, Sn.
It is believed that thin refractory flakes produced by any of the above-discussed methods would be too expensive to be put to any widespread use.